This invention relates generally to treatments for inhibiting thermal oxidation of polypropylene webs and more particularly a surface treatment for inhibiting thermal oxidation of polypropylene webs which can be applied to the polypropylene webs as a rinse after the polypropylene webs have been washed.
Webs made of polypropylene fibers are useful in a variety of applications in the home, in industry, and in the medical field. Such webs can be formed either of discrete microfibers by means of a melt-blown process as taught in U.S. Pat. No. 3,978,185 to Buntin, et al., or can be formed of continuous filaments by means of a spun-bond process as taught in U.S. Pat. No. 3,692,618 to Dorschner, et al. or U.S. Pat. No. 4,340,563 to Appel, et al., all of which patents are hereby incorporated by reference. Both the melt-blown process and the spun-bond process are well known in the art, and by varying the process parameters for each process, a wide variety of polypropylene webs can be made having characteristics particularly suitable to a large number of applications. In addition, additional characteristics for polypropylene webs can be achieved by laminating combinations of melt-blown and spun-bonded polypropylene webs as taught, for example, by U.S. Pat. No. 4,041,203 to Brock, et al. which is hereby incorporated by reference.
Polypropylene material is, however, unstable and will oxidize and decompose when exposed to air. As a result, stabilizers are conventionally blended with the polypropylene during the process of manufacturing the polypropylene beads which are used as the starting material for manufacturing polypropylene webs. Such internal stabilizers, however, are consumed in the act of stabilization and therefore, all webs composed of polypropylene will sooner or later decompose if exposed to air. Decomposition, of course, is faster at higher temperatures and with greater surface area. Thermal oxidative degradation of the polypropylene materials is further accelerated by washing the materials which tends to leach the internal stabilizers out of the microfibers.
In medical applications, polypropylene fabrics often have a very high surface to volume ratio and are often subjected to steam sterilization and drying for extended periods of time, all of which contributes to thermal oxidative degradation. In addition, polypropylene webs are precluded from some medical uses such as newborn baby blankets which are typically stored in warmers for extended periods.
We have found that the thermal oxidative degradation phenomenon cannot be satisfactorily inhibited by simply adding more internal stabilizers to the polymer before extrusion and forming into webs. It appears that such an approach fails for two reasons. First, the exponential decay of the internal stabilizers is so great that it is not physically possible to add enough of the stabilizers to the polymer beads. Second, higher levels of internal stabilizer cause yellowing of the polypropylene fabrics during shipment and storage.
We have also discovered that is virtually impossible to provide an effective internal stabilizer that will not be leached out of microfibers during laundering.